The solar system and seasons. Attnom Academy

S3.12.1 Chapter 1

The planets

Some bright dots in the night sky are planets, not stars. What is a planet?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A Four are small and rocky, two are giant balls of gas, and two are giant and icy.
You picked A. Read on. At the end, the chapter comes back to your guess.
B All eight planets are rocky balls, like the Earth.
You picked B. Read on. At the end, the chapter comes back to your guess.
C The eight planets are all the same size. Only their distance is different.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Name the eight planets in order from the Sun
Compare planets by size, distance and what they are made of

The Sun is a star. A planet is a big ball of rock or gas that goes round a star. Eight planets go round our Sun.

The Sun and everything that goes round it is called the solar system. The Earth is a planet, so you live in the solar system.

Draw it first

Draw the planets in order

1 Draw the Sun. It is the star in the middle.
2 Draw eight rings round the Sun. Put one planet on each ring.
3 Write the names in order. Start with the planet nearest the Sun.

In order from the Sun, the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

A sentence helps you remember the order. Use the first letters: M, V, E, M, J, S, U, N. Make your own sentence. Your own silly sentence will stay in your head best.

The picture is not to scale. The rings are spaced evenly so that all eight fit. In space the outer planets are far, far apart.

The four planets nearest the Sun are small and made of rock. They are rocky planets. The Earth is one of them. You can stand on a rocky planet.

Jupiter and Saturn are huge balls of gas. They are gas giants. They have no ground to stand on.

Uranus and Neptune are ice giants. They hold a lot of water and other icy stuff, under thick gas. They are not solid balls of ice.

See it three ways

As a picture

In words

The table lists the planets in order from the Sun. The four nearest are rocky. Then come two gas giants and two ice giants. The distances grow bigger and bigger, and the gaps between the outer planets are huge.

The planets are very different in size. Jupiter is the biggest. It is about 11 times as wide as the Earth. Mercury is the smallest.

Earth and Jupiter at one scale. Jupiter is drawn 11 times as wide as the Earth.

Distances in space are so large that we count them in millions of kilometres. The Earth is about 150 million km from the Sun. Neptune is about 4 500 million km away.

How far Earth and Mars are from the Sun. Read the two numbers for the next example.

Worked example

How much farther from the Sun is Mars than the Earth?

Earth is about 150 million km from the Sun.
Mars is about 228 million km from the Sun.
Take away the smaller from the bigger: 228 − 150 = 78.
Mars is about 78 million km farther from the Sun.

Answer: 78 million km

Try it

Draw Earth and Jupiter at one scale

You need: a big sheet of paper, a pencil, a string about 10 cm long, a ruler or a paper strip marked in cm.

Be safe: Keep the pencil point away from your face and eyes.

Let 1 cm stand for 10 000 km. The Earth is about 12 700 km wide, so it is about 1.3 cm wide.
Draw the Earth as a dot or a tiny circle, 1.3 cm wide.
Jupiter is about 140 000 km wide, so it is about 14 cm wide. Tie the pencil to one end of the string. Mark the string 7 cm from the pencil point.
Hold the mark on the paper with a finger. Pull the string tight and draw a full circle round your finger.
Lay your Earth beside Jupiter. How many Earths would fit across it?

What you will see: About 11 Earths fit across Jupiter. The planet that looks like a tiny dot beside Jupiter is the world you live on.

If you cannot try it, this is what happens.

A wrong idea many people have

All the planets are rocky balls, like the Earth.

Why it feels right: We live on rock, and pictures show every planet as a round ball with a surface.

The test that breaks it: Look at the table. Only four planets are rocky. Jupiter is a gas giant. A ship that tried to land would sink through thick gas. It would find no ground.

What is true: Only Mercury, Venus, Earth and Mars are rocky. The other four are giants, made mostly of gas or of water and icy stuff.

Key words

planet: a big ball of rock or gas that goes round a star.
solar system: the Sun and everything that goes round it.
rocky planets: small planets made of rock. There are four.
gas giants: huge planets made of gas. Jupiter and Saturn.
ice giants: huge planets with a lot of water and icy stuff. Uranus and Neptune.

Back to your guess

The answer is A: Four are small and rocky, two are giant balls of gas, and two are giant and icy.

The four planets nearest the Sun are small and rocky. Jupiter and Saturn are giant balls of gas. Uranus and Neptune are giants with a lot of water and icy stuff. The planets are very different in size.

Keep it

Copy this into your exercise book.

Eight planets go round the Sun. Rocky: Mercury, Venus, Earth, Mars. Gas giants: Jupiter, Saturn. Ice giants: Uranus, Neptune. Jupiter is biggest.

Draw: The Sun, and eight rings round it with the planets named in order.

S3.12.2 Chapter 2

How big and how far

How far apart are the planets? Could I make a model of them with a string?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A The planets are close together, like peas on a plate.
You picked A. Read on. At the end, the chapter comes back to your guess.
B The planets are far apart, but a model fits on a table.
You picked B. Read on. At the end, the chapter comes back to your guess.
C The planets are very far apart. A model needs a string as long as a playground.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Use a scale to turn real distances into model distances
Make a scale model of the planets along a long string

Space is far too big to draw as it is. So we make a model. A model is smaller than the real thing, but every part is in the right place.

To make a model, we choose a scale. A scale says how much real distance one piece of the model stands for.

Here is our scale. One metre of string stands for 100 million km. We write it as 1 m = 100 million km.

Draw it first

Put the planets on a string

1 Write the scale. 1 m of string stands for 100 million km. Put a stone for the Sun at one end.
2 Divide the real distance by 100. The Earth is 150 million km away, so 150 ÷ 100 = 1.5 m.
3 Do the same for every planet. Mark each one on the string.

The rule is short. The model distance is the real distance divided by the scale. Real distance ÷ scale = model distance.

The four rocky planets all fit on the first 2.5 m of string.

Now go out to Saturn, Uranus and Neptune. They are much farther out, so the string must be 50 m long. The scale and the rule stay the same.

Saturn, Uranus and Neptune on a string 50 m long. Jupiter would sit at 7.78 m, and the four rocky planets in the first 2.3 m.

See it three ways

As a picture

In words

Each row gives the real distance from the Sun and the model distance on the string. The metres are the million km divided by 100. Neptune goes 45 m out. Mercury goes 0.58 m, just over half a metre.

In symbols

Model distance = real distance ÷ 100. For Neptune: 4 500 ÷ 100 = 45 m.

Venus and Mars on the string. The next example finds the gap between them.

Worked example

On the model, 1 m stands for 100 million km. How far apart are Venus and Mars on the string?

Venus is about 108 million km from the Sun. Mars is about 228 million km.
Venus on the string: 108 ÷ 100 = 1.08 m.
Mars on the string: 228 ÷ 100 = 2.28 m.
Take away the smaller from the bigger: 2.28 − 1.08 = 1.2 m.

Answer: 1.2 m

Now think about the sizes of the planets. Use the same scale, 1 m = 100 million km. How wide are the Sun and the planets on the model?

The Sun would be a bead 1.4 cm wide. The Earth would be a speck of dust.

At this scale the Earth is only 0.13 mm wide. That is a speck you could hardly see. So a model of sizes and a model of distances need different scales. If you made the planets big enough to see, they would not fit on the string. If you used one scale for both, you could not see the planets.

Try it

Walk the solar system

You need: a long string or rope, or many strings tied together, 45 m long, a stick or a rope marked at 1 m, ten small stones, an open flat place, such as a playground or a field.

Be safe: Choose a flat place away from roads, holes and crowds. Do not leave string across a path where people walk.

Put a stone on the ground. It is the Sun.
Walk away from it, pulling the string tight. Measure each distance in the table with your 1 m stick.
Put a stone at 0.58 m for Mercury and 1.08 m for Venus. Put one at 1.5 m for Earth and 2.28 m for Mars. They are very close.
Walk on. Put stones at 7.78 m for Jupiter and 14.3 m for Saturn. Then 28.7 m for Uranus and 45 m for Neptune.
Stand at Neptune. Look back at the Sun stone. If 45 m is too long, use 1 m = 200 million km and halve every number.

What you will see: The four rocky planets bunch up near the Sun, in about 2 m of string. Then there is a long, long gap before Jupiter. Neptune stands 45 m from the Sun. Most of the solar system is empty space.

If you cannot try it, this is what happens.

A wrong idea many people have

A good model of the solar system shows the planets as big balls close together.

Why it feels right: Pictures in books squeeze everything onto one page, so the planets look like neighbours on a shelf.

The test that breaks it: Put the planets on the string. Mars is at 2.28 m and Jupiter is at 7.78 m. The gap is more than 5 m. At the same scale the Earth is a speck of dust.

What is true: The planets are tiny compared with the gaps between them. A picture in a book shows them big and close so you can see them. It is not to scale.

Key words

scale: how much real distance one piece of the model stands for.
model distance: the distance on the model, in metres.
real distance: the true distance in space, in million km.

Back to your guess

The answer is C: The planets are very far apart. A model needs a string as long as a playground.

The planets are very far apart. If 1 m of string stands for 100 million km, Neptune is 45 m from the Sun. That is a long walk.

Keep it

Copy this into your exercise book.

A scale says how much real distance a piece of the model stands for. Model distance = real distance ÷ scale. 1 m = 100 million km puts Neptune 45 m out.

Draw: A string with a Sun at 0, the Earth at 1.5 m and Neptune at 45 m. Write 1 m = 100 million km.

S3.12.3 Chapter 3

Gravity and orbits

Why does the Earth not fall into the Sun, or fly off into space?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A Nothing holds the Earth. It just floats there.
You picked A. Read on. At the end, the chapter comes back to your guess.
B The Sun pulls the Earth, and the Earth is moving sideways, so it keeps curving round the Sun.
You picked B. Read on. At the end, the chapter comes back to your guess.
C A hard, invisible wall keeps the Earth on its path.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Explain that gravity keeps planets and moons in orbit
Explain why planets far from the Sun take longer to go round

The Earth goes round the Sun. The Moon goes round the Earth. What keeps them on their paths?

Gravity is the pull that every body has on every other body. The more mass a body has, the stronger its pull. The Sun is huge. It holds nearly all the mass of the solar system.

So the Sun pulls on every planet. The pull is strong enough to hold the Earth, even from 150 million km away.

If the pull were all, the planets would fall straight into the Sun. But each planet is also moving sideways, very fast. The Earth moves about 30 km every second.

Draw it first

Draw why a planet curves

1 Draw the Sun and a planet. Draw an arrow from the planet to the Sun. It is the pull of gravity.
2 Draw a second arrow from the planet, at right angles to the first. It shows the planet moving sideways.
3 The pull keeps bending the path. The planet follows a curve round the Sun. Draw the curve and call it the orbit.

The curved path round a bigger body is an orbit. The pull never lets the planet go, and the sideways motion never lets it fall in. So it goes round and round.

Without the pull, a planet would not go round. It would fly off in a straight line. You can see this with a ball on a string.

Try it

Swing a ball on a string

You need: a light, soft ball made of rolled cloth or a knotted sock, a string about 1 m long, a wide, open, flat place with nobody near.

Be safe: Use only a light, soft ball. Never use a stone. Swing and let go well away from people, animals and windows.

Tie the string firmly to the soft ball. Check that nobody is within 4 m.
Hold the end of the string. Swing the ball round in a flat circle at about waist height.
Feel the string pull your hand inwards. The pull is like gravity.
Slow down and stop. Then swing again and let go on purpose, with nobody in the way. Watch where the ball goes.

What you will see: While you hold the string, the ball goes round in a circle. The string pulls it inwards all the time. When you let go, the pull stops. The ball flies off in a straight line, not round and round.

If you cannot try it, this is what happens.

The Sun is like your hand, and gravity is like the pull of the string. Let the pull go, and a planet leaves its orbit in a straight line.

Gravity also holds moons. The Earth pulls on the Moon, and the Moon moves sideways, so the Moon goes round the Earth.

The Earth pulls the Moon inwards. The Moon moves sideways. So it goes round the Earth.

Now think about time. One trip round the Sun is a year. The Earth takes about 365 days, so our year has 365 days.

A planet that is nearer the Sun has a shorter way round, and it moves faster. A planet that is farther out has a longer way round, and it moves slower.

See it three ways

As a picture

In words

The nearer a planet is to the Sun, the shorter its orbit and the faster it moves. So Mercury takes the shortest time to go round. It takes only 88 days. The Earth takes 365 days.

In symbols

Mercury: 88 days. Venus: 225 days. Earth: 365 days.

The year of each rocky planet, in Earth days. The farther the planet, the longer the year.

The year of each giant, in Earth years. Neptune takes 165 Earth years to go round once.

There are two reasons. A far planet has a longer way round. It also moves more slowly, because the Sun pulls less far out. The Earth moves about 30 km a second. Neptune moves only about 5 km a second.

Worked example

Venus takes 225 days to go round the Sun. How many times does it go round in 900 Earth days?

One trip takes 225 days.
Two trips take 2 × 225 = 450 days.
Four trips take 4 × 225 = 900 days.
So 900 ÷ 225 = 4. Venus goes round 4 times.

Answer: 4 times

A wrong idea many people have

There is nothing to hold a planet, so it just floats where it is.

Why it feels right: Space looks empty, and there are no ropes or walls to see.

The test that breaks it: Swing the ball on its string. The ball goes round only while the string pulls. Let go, and it flies away. A planet stays on its path because something pulls it all the time.

What is true: Gravity is the invisible pull. It bends the path of each planet into an orbit round the Sun.

Key words

gravity: the pull that every body has on every other body.
orbit: the curved path of one body going round another.
year: the time a planet takes to go round the Sun once.

Back to your guess

The answer is B: The Sun pulls the Earth, and the Earth is moving sideways, so it keeps curving round the Sun.

The Sun pulls the Earth with gravity. The Earth is also moving sideways. The pull bends its path into a curve, so the Earth keeps going round the Sun.

Keep it

Copy this into your exercise book.

Gravity pulls planets towards the Sun. A planet also moves sideways, so its path curves into an orbit. Planets far out go slower and have a longer year.

Draw: The Sun and a planet. Add an arrow of pull to the Sun, an arrow sideways, and a curved orbit.

S3.12.4 Chapter 4

Tilt and the seasons

Why is it hot for part of the year and cold for another part? Is the Earth nearer the Sun in summer?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A The Earth is nearer the Sun in summer and farther away in winter.
You picked A. Read on. At the end, the chapter comes back to your guess.
B The Sun itself gets hotter and colder through the year.
You picked B. Read on. At the end, the chapter comes back to your guess.
C The Earth's axis is tilted, so each half takes turns leaning towards the Sun.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Explain seasons using the tilt of the Earth's axis
Model seasons with a tilted ball going round a lamp

Through the year the weather changes. So does the length of the day. A season is a part of the year with its own kind of weather.

The Earth spins round a line through its middle, from the North Pole to the South Pole. This line is the axis.

The axis does not stand upright. It leans over by about 23.4 degrees. We say the Earth is tilted.

Draw it first

Draw the tilted Earth

1 Draw the Sun and the Earth. Draw an upright dotted line through the Earth, to show what upright means.
2 Draw the axis leaning by 23.4°. Write N at the north end and S at the south end.
3 Draw the sunlight going from the Sun to the Earth. The north end of the axis leans towards the Sun.

The axis points the same way in space all year. It always points to the same far-off spot in the sky. The Earth goes round the Sun, but the axis does not turn.

The Earth at four places on its path. The axis leans the same way each time. N shows the north end.

The Earth has two halves. The half north of the equator is the northern hemisphere. The half south of it is the southern hemisphere.

At A, the northern hemisphere leans towards the Sun. It has summer. The southern hemisphere leans away. It has winter.

Half a year later the Earth is at C. Now the south leans towards the Sun and has summer. The north leans away and has winter.

The north is tipped towards the Sun. The north has summer. The rings across the Earth are lines of latitude. The middle one is the equator.

Why does the tip make the difference? There are two reasons. The Sun climbs higher in the sky at midday. And the day is longer.

A high Sun is better at warming. Look at one beam of light, 10 cm wide, from a high Sun and from a low one.

See it three ways

As a picture

In words

The same beam of light covers more ground when the Sun is low. So each square centimetre gets less light and less warmth. A high Sun gives a stronger warming.

In symbols

At 30° the patch is 10 × 2 = 20 cm long. The light is spread twice as thin.

Away from the equator, in summer the Sun is high at midday, and the days are long. So the ground has more time to warm, and it warms faster. In winter the Sun is low and the days are short.

The Earth does not change in distance very much. The table shows how far it is from the Sun at its nearest and its farthest.

The Earth is nearest the Sun in early January. It is farthest in early July.

The difference is only about 5 million km out of 150 million km. That is tiny. And in early January it is winter in the north and summer in the south.

Try it

Make seasons with a ball and a lamp

You need: a torch or battery lamp, a ball or an orange, a stick or pencil to push through the ball, a pen, a table in a room you can make dark, a sticky note for the wall.

Be safe: A lamp can get hot. Keep it away from paper and do not touch the bulb. Never use a lamp plugged into a wall socket. Never shine a torch into anyone's eyes.

Push the stick through the ball from top to bottom. The ends are the poles. Lean the stick over, about a quarter of the way to flat.
Draw a dot on the upper half of the ball for the north. Draw another on the lower half for the south.
Put the lamp in the middle of the table. Stick the note on one wall of the room.
Carry the ball in a big circle round the lamp. Keep the top of the stick leaning towards the note all the way. Never turn it.
Stop at four places, a quarter of the way apart. At each place, see which dot gets more light.

What you will see: In one place the top of the stick leans towards the lamp. The north dot gets more light. Half way round it leans away, and the south dot gets more light. In the two places between, both dots are lit about the same.

If you cannot try it, this is what happens.

A wrong idea many people have

It is summer when the Earth is nearer the Sun.

Why it feels right: A fire feels hotter when you are nearer. So the Sun seems to work the same way.

The test that breaks it: The Earth is nearest the Sun in early January. But it is winter in the north then. When the north has summer, the south has winter, yet both are the same distance from the Sun.

What is true: The tilt causes the seasons. The half of the Earth tipped towards the Sun has summer. The distance changes very little.

Key words

season: a part of the year with its own kind of weather.
axis: the line through the Earth from pole to pole, which it spins round.
tilted: leaning over. The Earth's axis leans by about 23.4 degrees.
hemisphere: one half of the Earth: north or south of the equator.
summer: the season when your hemisphere is tipped towards the Sun.
winter: the season when your hemisphere is tipped away from the Sun.

Back to your guess

The answer is C: The Earth's axis is tilted, so each half takes turns leaning towards the Sun.

The Earth's axis is tilted. The half that leans towards the Sun has summer. The half that leans away has winter. The distance to the Sun hardly changes.

Keep it

Copy this into your exercise book.

The Earth's axis is tilted about 23.4° and points the same way all year. The half tipped towards the Sun has summer. The other half has winter.

Draw: The Sun, and the Earth at two opposite places with the axis tilted the same way. Mark summer and winter in each half.

S3.12.5 Chapter 5

Seasons around the world

Why is it summer in one place while it is winter in another? Why is it warm all year at the equator?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A Every place on Earth has the same season at the same time.
You picked A. Read on. At the end, the chapter comes back to your guess.
B The south is always farther from the Sun than the north.
You picked B. Read on. At the end, the chapter comes back to your guess.
C When the north leans towards the Sun, the south leans away. So the two halves have opposite seasons.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Explain why the two hemispheres have opposite seasons
Explain why places near the equator stay warm all year
Explain how the rain belt follows the overhead Sun to make wet seasons

In some places the year goes from hot to cold and back again. Near the equator it stays warm. There the seasons are wet and dry. Why is the Earth so different from place to place?

First, the two halves of the Earth. In the last chapter you saw that the north and the south take turns leaning towards the Sun. When one half leans in, the other leans out.

The south is tipped towards the Sun. The south has summer and the north has winter.

When the south leans towards the Sun, it has summer. At the same time the north leans away and has winter. When the north has its longest day, the south has its shortest. The two halves have opposite seasons.

Now look at the middle of the Earth. The equator is the line round the middle, halfway between the poles. Places near it are warm all year.

At the equator the Sun is always high at midday. Even on its lowest day it is more than 66 degrees above the horizon. That is not far from straight above you.

The lowest the midday Sun gets at the equator. It is still very high, so the shadow is short.

The days near the equator are about the same length all year. The table gives the hours of day at the equator. It is about 12 hours of day and 12 hours of night, in every month.

See it three ways

As a picture

In words

Near the equator the Sun is high at midday. The day is about 12 hours long all year. The ground gets strong sunlight for a long time every day, so it stays warm.

In symbols

At the equator: 12 hours of day + 12 hours of night = 24 hours.

There is more. Between two special lines the midday Sun can stand straight overhead. Overhead means straight above you. One is the Tropic of Cancer, 23.4 degrees north of the equator. The other is the Tropic of Capricorn, 23.4 degrees south of it.

The band between the two lines is called the tropics. Outside the tropics the midday Sun is never overhead.

The place where the midday Sun is overhead moves. It goes north to the Tropic of Cancer and back south. It crosses the equator on the way. Then it goes to the Tropic of Capricorn and back. This takes one year.

The overhead Sun on four days of the year. It goes north, then south, then north again.

Now rain. Where the Sun is high, the ground gets very hot. The hot ground warms the air above it. Warm, moist air rises. As it rises it cools, and clouds and heavy rain form.

So rain falls most under the overhead Sun. There is a belt of cloud and rain round the Earth near the equator. It is the rain belt. It follows the overhead Sun north and south, a little behind it.

Draw it first

Draw the rain belt following the Sun

1 Draw the Earth with the equator and the two tropics. When the Sun is overhead at the equator, draw the rain belt along the equator.
2 Move the overhead line to the Tropic of Cancer. The belt moves north with it.
3 Move the overhead line to the Tropic of Capricorn. The belt moves south.

These pictures show the idea. The real belt is not a neat band. Land, sea and mountains push it about. We say it moves roughly with the overhead Sun.

When the rain belt is over a place, it is the wet season. When the belt has moved away, it is the dry season. The belt passes over a place near the equator twice a year. That place has two wet seasons. A place farther from the equator gets it once a year, so it has one wet season. Some places near the tropics are past where the belt reaches. They stay dry, and many are deserts.

Try it

Move the overhead Sun with a torch

You need: a torch or battery lamp, a ball or an orange, a stick or pencil to push through the ball, a pen, a room you can make dark.

Be safe: A lamp can get hot. Keep it away from paper and do not touch the bulb. Never use a lamp plugged into a wall socket. Never shine a torch into anyone's eyes.

Push the stick through the ball from top to bottom. Draw the equator round the middle with the pen.
Draw one line about a quarter of the way up from the equator, for the Tropic of Cancer. Draw one the same distance down, for the Tropic of Capricorn.
In the dark, hold the torch a hand's width from the ball. Shine it at the equator. The brightest spot is where the Sun is overhead.
Lean the top of the stick towards the torch, about a quarter of the way to flat. Keep the torch still, shining at the middle of the ball. Watch the bright spot.
Lean the stick the other way, so the bottom leans towards the torch. Watch the spot again.

What you will see: The bright spot is on the equator when the stick is upright. It moves north when you lean the top towards the torch. It moves south when you lean the other way. It never goes beyond the two tropic lines.

If you cannot try it, this is what happens.

A wrong idea many people have

The equator is warm because it is nearer the Sun.

Why it feels right: A map or a ball shows the equator sticking out towards the Sun. Nearer things feel hotter.

The test that breaks it: The side of the Earth that faces the Sun is at most about 6 400 km nearer. The Sun is 150 million km away. Such a tiny difference cannot matter.

What is true: The equator is warm because the midday Sun is always high there. The days are about 12 hours long all year. The sunlight is strong and it lasts.

Key words

equator: the line round the middle of the Earth, halfway between the poles.
overhead: straight above you.
tropics: the band between the Tropic of Cancer and the Tropic of Capricorn.
rain belt: the belt of cloud and heavy rain that follows the overhead Sun.
wet season: the time when the rain belt is over a place.
dry season: the time when the rain belt has moved away.

Back to your guess

The answer is C: When the north leans towards the Sun, the south leans away. So the two halves have opposite seasons.

The axis leans one way. When the north is tipped towards the Sun, the south is tipped away. So the two halves have opposite seasons, at the same time.

Keep it

Copy this into your exercise book.

North and south have opposite seasons. Near the equator the Sun is always high, so it stays warm. The rain belt follows the overhead Sun and makes the wet seasons.

Draw: The Earth with the equator and two tropics. Draw the overhead line and the rain belt on it.

S3.12.6 Chapter 6

Day length and the Sun's height

Why are some days longer than others? Can I track the Sun with a stick?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A Every day has the same length everywhere, all year.
You picked A. Read on. At the end, the chapter comes back to your guess.
B The days change because the Earth spins faster in summer.
You picked B. Read on. At the end, the chapter comes back to your guess.
C In most places the day length changes through the year. Near the equator it hardly changes.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Graph day length through a year from a table
Track the Sun's midday height by measuring a stick's shadow

The day length is the time from sunrise to sunset. It is the hours of daylight. In most places it is not the same all year.

Near the equator, day and night are about 12 hours each, all year. Farther from the equator the day length changes a lot.

Here are the day lengths for two places. One is 40 degrees north of the equator. The other is 40 degrees south of it. The table gives the hours of day in the middle of each month.

See it three ways

As a picture

In words

At 40° north the day is longest in June and shortest in December. At 40° south it is the other way round. The two places have the same pattern, six months apart.

In symbols

40° N: longest 14.8 hours, shortest 9.2 hours.

Draw it first

Graph the day length

1 Draw the two axes. Put the month number along the bottom and the hours of day up the side. Plot the first four months.
2 Plot the next four months. Put each point exactly at its month and its hours.
3 Plot the last four months. Join the points with a line.

The line rises to a peak, then falls to a low point, then rises again. At 40° north the peak is in the middle of the year.

Day length at 40° north. The peak is the longest day. The low point is the shortest.

Day length at 40° south. The peak is at the end of the year, six months from the north.

Worked example

Use the graph for 40° north. How much longer is the longest day than the shortest day?

The graph for 40° north has one peak and one low point in the year.
The peak is at month 6, June.
At month 6 the graph is at 14.8 hours. That is the longest day.
The lowest point, at month 12, is 9.2 hours. That is the shortest day.
Take away the smaller from the bigger: 14.8 − 9.2 = 5.6 hours.

Answer: 5.6 hours

Near the poles the changes are huge. Inside the polar circles some days have 24 hours of light. Others have 24 hours of dark. At the poles themselves the Sun stays up for about six months. Then it stays down for about six months.

The ring round the north pole is in the light all day. The ring round the south pole is in the dark all day.

Now the Sun's height. At midday the Sun is at its highest point in the sky. The higher it is, the shorter the shadow of a stick. A low Sun makes a long shadow.

A stick 100 cm tall. A higher Sun gives a shorter shadow. A stick half as tall gives half the shadow.

A stick that is twice as tall has a shadow twice as long. Later in mathematics you will meet a rule for this, using a button called tan. You do not need it now. The table gives the numbers.

The shadow points away from the Sun. So which way it points at midday depends on where you live.

At 40° north the midday Sun is in the south, so the shadow points north. This is on a day when the midday Sun is overhead at the equator.

At 40° south the midday Sun is in the north, so the shadow points south. This is on a day when the midday Sun is overhead at the equator.

North of the Tropic of Cancer the midday shadow always points north. South of the Tropic of Capricorn it always points south. Between the two tropics it can point either way. On two days a year the Sun is overhead, and there is no shadow at all.

Try it

Track the midday Sun with a stick

You need: a straight stick about 1 m tall, a flat patch of ground, in the Sun all day, small stones, a ruler or a string marked in cm, a clock or a watch, paper and a pencil.

Be safe: Look only at the ground and at the shadow. Never look at the Sun, not even for a moment, and not through anything.

Push the stick into the ground, so that it stands straight up. Press the earth round it. Measure its height above the ground.
Start at about 11 by the clock and go on until about 2. Every 15 minutes, put a stone on the tip of the shadow.
The shortest shadow is the midday one. Measure it from the foot of the stick to the stone, in cm.
Do it once a week for four weeks. Write down each midday shadow.
Look at the numbers. Is the shadow getting longer, or shorter?

What you will see: Our shadows were 70, 76, 82 and 90 cm long. They grew longer, so the Sun was getting lower at midday. Your shadows may get shorter, if the Sun is getting higher where you live.

If you cannot try it, this is what happens.

A wrong idea many people have

The shortest shadow comes when the clock says 12.

Why it feels right: The word midday sounds like 12 o'clock, so the Sun should be highest then.

The test that breaks it: Mark the tip of the shadow every 15 minutes. The shortest shadow may come before or after 12 on the clock, sometimes by an hour or more. Clocks are set for a whole region, not for your exact place.

What is true: The Sun is highest at midday, and then the shadow is shortest. That is true midday, whatever the clock says.

Key words

day length: the time from sunrise to sunset, in hours.
sunrise: the moment the Sun first shows over the horizon.
sunset: the moment the Sun disappears below the horizon.

Back to your guess

The answer is C: In most places the day length changes through the year. Near the equator it hardly changes.

The tilt makes day length change through the year. The change grows as you go away from the equator. At the equator it is close to 12 hours all year. The Earth spins at the same speed all year.

Keep it

Copy this into your exercise book.

Day length changes through the year, except near the equator. North and south have opposite patterns. A high midday Sun gives a short shadow, pointing away from the Sun.

Draw: A graph of day length against month, rising and falling once. Add a stick with a short shadow.

S3.12.7 Chapter 7

Moons, asteroids and comets

A shooting star flashes across the sky. Is it a star that is falling?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A It is a small bit of rock or dust burning up high in the air.
You picked A. Read on. At the end, the chapter comes back to your guess.
B It is a star that has fallen out of the sky.
You picked B. Read on. At the end, the chapter comes back to your guess.
C It is a planet that has lost its way.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Describe moons, dwarf planets, asteroids and comets
Explain what a shooting star really is

The solar system holds more than the Sun and the eight planets. It holds moons, dwarf planets, asteroids and comets. Each is a different kind of body.

A moon is a body that goes round a planet or a dwarf planet. The Earth has one Moon. Mercury and Venus have none. Mars has two.

The giant planets have many moons. New ones are still being found, so we say about.

Moons of each planet. Saturn has the most, about 270.

A dwarf planet is round, like a planet. But it shares its path round the Sun with many other bodies. Pluto and Ceres are dwarf planets.

Dwarf planets are small. Pluto is smaller than our Moon. The table gives the width of some bodies.

Pluto and Ceres are dwarf planets. Both are smaller than the Moon.

An asteroid is a lump of rock that goes round the Sun. Most asteroids lie in a belt between Mars and Jupiter. There are millions of them, and most are small.

The asteroid belt lies between Mars and Jupiter. Not to scale.

A comet is a ball of ice and dust on a long, thin orbit round the Sun. Far from the Sun it is a frozen lump with no tail.

Near the Sun the comet warms. Gas and dust stream off it. The Sun's light and wind push them away from the Sun, so the comet gets a tail. The tail always points away from the Sun.

Draw it first

Draw a comet on its long orbit

1 Draw the Sun and a long, thin orbit. Draw the comet far away with no tail, and a little nearer with a short tail.
2 Draw the comet near the Sun, too. Near the Sun its tail is longest.
3 Draw each tail pointing straight away from the Sun. Name the parts.

A comet's tail points away from the Sun even when the comet is moving away. The tail is not smoke blown out behind it. It is pushed by the Sun.

Now, the shooting star. Tiny bits of rock and dust fly through space. Many are smaller than a pea. Sometimes one meets the Earth and flies into our air.

The bit moves at tens of kilometres every second. It squeezes the air in front of it. The air gets very hot. The bit glows and burns up, high above the ground, about 100 km up.

See it three ways

As a picture

In words

A small bit of rock or dust burns up in the air. The glowing streak is called a meteor. People call it a shooting star, but it is not a star. A real star is a far-off sun.

We call the streak of light a meteor. Most bits burn up completely, so nothing reaches the ground. If a piece does reach the ground, it is a meteorite.

Try it

Watch for shooting stars

You need: an adult to come with you, a warm blanket or mat to lie on, a safe, open place with a clear dark sky, paper and a pencil.

Be safe: Go with an adult, to a safe place away from roads. Never go alone at night. Stay on the mat and do not wander in the dark.

Go out at night with an adult. Choose a safe place, away from roads, water and steep edges.
Lie on the mat and look at the sky. Wait for ten minutes, so your eyes get used to the dark.
Watch for 30 minutes. Every time you see a bright streak, make a mark on your paper.
After each 10 minutes, count the marks and write the number down.

What you will see: You may see a few streaks, or none. Some nights are better than others. Each streak is a bit of rock or dust burning up, high in the air.

If you cannot try it, this is what happens.

A wrong idea many people have

A shooting star is a star that falls to the Earth.

Why it feels right: The streak looks like a star that has come loose and dropped across the sky.

The test that breaks it: The stars are still in their places after a streak has gone. A real star is a sun, very far away. It does not drop across the sky.

What is true: A shooting star is a meteor. It is a small bit of rock or dust that burns up high in the air.

Key words

moon: a body that goes round a planet or a dwarf planet.
dwarf planet: a small, round body that shares its path with many others.
asteroid: a lump of rock that goes round the Sun, mostly in a belt between Mars and Jupiter.
comet: a ball of ice and dust on a long orbit, with a tail near the Sun.
meteor: the streak of light when a bit of rock or dust burns up in the air.
meteorite: a piece of space rock that reaches the ground.

Back to your guess

The answer is A: It is a small bit of rock or dust burning up high in the air.

A shooting star is a small bit of rock or dust from space. It burns up in the air high above us. A real star is a huge, far-off sun, and it does not fall.

Keep it

Copy this into your exercise book.

Moons orbit planets. Asteroids are rocks, mostly between Mars and Jupiter. Comets are icy and their tails point away from the Sun. A shooting star is a meteor, burning up.

Draw: The Sun and a long thin orbit. Draw a comet near the Sun, its tail pointing away.

S3.12.8 Chapter 8

Exploring space

Nobody has been to Jupiter. How do we know what it looks like?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A People have visited every planet and taken photographs.
You picked A. Read on. At the end, the chapter comes back to your guess.
B Telescopes, satellites and space probes collect light and send us pictures and measurements.
You picked B. Read on. At the end, the chapter comes back to your guess.
C We only guess, from old stories.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Explain how telescopes, satellites and space probes help us learn
Name ways satellites help people, such as weather and phone signals

People have walked on only one other body, the Moon. No person has been to another planet. Yet we know what Jupiter's clouds look like. Three kinds of tool do the visiting for us.

The first tool is the telescope. A telescope collects more light than your eye can. So it shows things that are faint or far away.

Draw it first

Draw how a telescope collects light

1 Draw a far star and six arrows of light coming from it. The star is so far away that the arrows run side by side.
2 Draw your eye as a small opening. It catches only two of the six arrows.
3 Draw a telescope as a wide opening. It catches all six arrows. A round opening 3 times as wide catches 9 times as much light.

A telescope has a big lens or mirror to catch the light. The wider it is, the more light it catches. More light means a brighter picture, so fainter and farther things can be seen.

Try it

Catch rice like a telescope catches light

You need: a small cup about 5 cm wide, a wide bowl about 15 cm wide, a tray or a cloth to work on, a handful of dry rice or small beans, paper and a pencil.

Be safe: Do not eat the rice. Sweep up any that falls on the floor, so that nobody slips.

Put the cup and the bowl side by side on the tray.
Hold a pinch of rice high above them. Let it fall like rain, evenly over both.
Do this until the cup has caught a few grains. Stop.
Count the grains in the cup and the grains in the bowl. Write both numbers down.
Divide the bowl's number by the cup's number. How many times as much did the bowl catch?

What you will see: The bowl is 3 times as wide as the cup. But it catches about 9 times as much rice. A wide mirror catches far more light, in the same way.

If you cannot try it, this is what happens.

The second tool is the satellite. A satellite is a body that goes round a planet. The Moon is a natural satellite of the Earth. People have also built satellites and sent them up. They orbit the Earth, held by gravity.

A satellite goes round the Earth on its orbit, held by gravity.

Satellites help us in many ways. They send down pictures of clouds, so we get weather forecasts. They carry phone and television signals across the world. They help a phone find where you are. They look at forests, farms, ice and the sea.

Four jobs for satellites that orbit the Earth.

The third tool is the space probe. A space probe is a machine sent to travel to another planet or moon. It has cameras and other instruments. It sends pictures and measurements back to the Earth by radio.

Some probes fly past a planet. Some go into orbit round it. Some land on it. No person has to go.

See it three ways

As a picture

In words

A telescope stays on the ground, or in orbit, and collects light. A satellite goes round the Earth. A space probe leaves the Earth and goes to another planet.

The probe's messages travel as radio waves. Radio waves go about 300 000 km every second. That is very fast, but space is very big. A signal needs time to cross it.

Radio and light go 300 000 km every second. Sunlight takes about 8 minutes to reach us.

Time = distance ÷ 300 000 km each second. A message from the Moon takes about 1.3 seconds. A message from a probe far out in the solar system can take hours.

A wrong idea many people have

We know about the planets because people have visited them.

Why it feels right: Pictures of planets look like photographs taken by a traveller.

The test that breaks it: Ask who took the picture. It was taken by a camera on a probe or on a satellite. Or it came from a telescope. No person was there.

What is true: We learn about the planets from light collected by telescopes. We also learn from probes and satellites that go there for us.

Key words

telescope: a tool that collects more light than your eye, to show faint and far things.
satellite: a body that goes round a planet, such as a built one that orbits the Earth.
space probe: a machine sent to travel to another planet or moon.

Back to your guess

The answer is B: Telescopes, satellites and space probes collect light and send us pictures and measurements.

Telescopes collect light from far away. Satellites orbit the Earth and look down. Space probes travel to other planets and send pictures and measurements home.

Keep it

Copy this into your exercise book.

Telescopes collect light to show far things. Satellites orbit the Earth for weather, signals and positions. Space probes travel to other planets and send messages by radio.

Draw: The Earth with a telescope, a satellite on a ring and a probe on an arrow.

Draw the planets in order

Draw Earth and Jupiter at one scale

Put the planets on a string

Walk the solar system

Draw why a planet curves

Swing a ball on a string

Draw the tilted Earth

Make seasons with a ball and a lamp

Draw the rain belt following the Sun

Move the overhead Sun with a torch

Graph the day length

Track the midday Sun with a stick

Draw a comet on its long orbit

Watch for shooting stars

Draw how a telescope collects light

Catch rice like a telescope catches light

Key words, A to Z